[libvirt] Add new API for accessing remote guest text console
The 'virsh console' command has been an oddity that only works
when run locally, as the same UID as the QEMU instance. This
is because it directly opens /dev/pty/XXX. This introduces a
formal API for accessing consoles that uses the virStreamPtr
APIs. Now any app can open consoles anywhere it can connect
to libvirt